OD ESR LINE INVERSION IN VISCOUS SOLUTION - INDICATION OF ST MECHANISM OF RADICAL-ION PAIR RECOMBINATION

The optically detected ESR spectra (OD ESR) of naphthalene radical ion pairs in squalane show inversion near the glass formation point of th e solvent. A similar inversion of OD ESR spectra is also observed for some other additives, such as biphenyl and tetrafluorobenzene in glass y nonpolar solvents (squalane, 3-methylpentane). A theory of radical p airs can be used to account for the observed inversion of signal ampli tude as a result of adiabatic passage of the anticrossing point of rad ical pair S and T-levels upon counter ion recombination. The compariso n between the shapes of experimentally observed and theoretically calc ulated spectra qualitatively confirm this hypothesis.